1. Field of the Invention
The present invention relates to equipment for telecommunication systems and, more specifically but not exclusively, to radio-frequency (RF) transmitters and power amplifiers, and passive RF circuits suitable for use therein.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A recent trend in the telecommunications industry includes the introduction of wideband digital-modulation systems, such as the third generation (3G) cellular-system wideband code-division-multiple-access (WCDMA) and the fourth generation (4G) cellular-system orthogonal frequency-division multiple-access (OFDMA). This trend has had a profound effect on power-amplifier specifications because an RF power amplifier used in a wideband digital-modulation system needs to properly and efficiently handle a signal that has a fast-changing envelope, a high peak-to-average power ratio (PAPR), and a bandwidth that can be tens of megahertz. In addition, for cost reasons, a single power amplifier is usually configured to amplify multiple modulated carriers.
A typical RF power amplifier is inherently nonlinear, with its gain being a function of the output-power level. The gain usually decreases when the output power approaches the saturation level of the amplifier, and the phase of the gain can either increase or decrease, depending on the type of the active medium. Amplitude and/or phase distortions in the power amplifier tend to cause the generation of spurious spectral components often referred to as inter-modulation-distortion (IMD) products. IMD products are detrimental, for example, because they increase the level of interference between adjacent RF channels.